Disc drives are data storage devices that store digital data on a rotating storage medium on a disc. Modern disc drives typically comprise one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Information is stored on the discs in a plurality of tracks typically by an array of transducers (“heads”) mounted to a radial actuator or actuator arm for movement of the heads relative to the discs. Transducers are used to transfer data between a desired track and an external environment. During a write operation, sequential data is written onto the disc track and during a read operation, the head senses the data previously written onto the disc track and transfers the information to the external environment. Important to both of these operations is the accurate and efficient positioning of the head relative to the center of the desired track. Head positioning within a desired track is dependent on head-positioning servo-patterns, i.e., a pattern of data bits used to maintain optimum track spacing and sector timing. Servo-patterns can be recorded between the data sectors on each track of a disc, termed embedded servo, or on one dedicated surface of a disc within the disc drive, termed dedicated servo.
Traditionally, the servo patterns are written onto the disc in-situ (i.e., after the disc is mounted on the spindle motor of a disc drive) when the disc drive is manufactured and are thereafter simply read by the disc drive to determine position.
Recent advances in disc drive manufacturing have created a need for a disc drive with the servo patterns written, or pre-written, to the discs prior to assembly of the disc stack. In particular some manufacturing efficiency has been realized by pre-writing the servo patterns to the disc during the manufacturing of the discs themselves.
Thus, there is a need for minimizing a number of pre-written tracks/cylinders (hereinafter referred to as greycode tracks/cylinders) that are inaccessible to heads of a disc drive, in which pre-written discs are installed, due to problems such as centering misalignment between tracks of the pre-written discs and the disc drive spindle.